1. Field of the Invention
This invention relates to a radiation image read-out apparatus wherein a stimulable phosphor sheet, on which a radiation image of an object has been stored, is exposed to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, and the emitted light is photoelectrically detected and converted into an image signal representing the radiation image. This invention also relates to a method for recording a radiation image on a stimulable phosphor sheet such that the range of the amount of light emitted by the stimulable phosphor sheet, on which a radiation image has been stored, can be compressed, and an apparatus for carrying out the method. This invention further relates to a stimulable phosphor sheet, which is used in the radiation image recording method and apparatus, and a cassette which houses the stimulable phosphor sheet.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395 it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as a human body. In this manner, a radiation image of the object is stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The electric image signal is then processed as desired. The processed image signal is used during the reproduction of a visible image which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. The visible image finally obtained may be reproduced as a hard copy or may be displayed on a cathode ray tube (CRT).
The radiation image recording and reproducing systems described above are applicable not only to medical diagnoses but also to detection of flaws in industrial products. Specifically, a radiation image of an industrial product, such as a round bar or a steel pipe, may be stored on a stimulable phosphor sheet and may then be reproduced as a visible image in the manner described above. In the visible image, even a flaw, which is present at a position deep from the surface of the industrial product, appears clearly as a pattern darker than the surrounding areas because of its lower radiation absorptivity.
Industrial metal products, such as round bars, pipes (e.g. steel pipes), and die steel products, have markedly higher radiation absorptivity than human bodies, or the like. Therefore, when a radiation image of such an industrial product is to be stored on a stimulable phosphor sheet, it is necessary for the industrial product to be exposed to a markedly higher dose of radiation than when a radiation image of a human body, or the like, is to be recorded. As a result, the amount of energy stored on the stimulable phosphor sheet during its exposure to the radiation differ markedly for the region corresponding to the part of the industrial product having a comparatively high radiation absorptivity and for the region corresponding to the part of the industrial product having a comparatively low radiation absorptivity.
When the radiation image of an industrial product is read out from the stimulable phosphor sheet, the dynamic range of the amount of light emitted by the stimulable phosphor sheet often becomes wider than 10.sup.4. However, the read-out range of a photoelectric read-out means, such as a photomultiplier, i.e. the range of the amount of light, which range the photoelectric read-out means can detect, is ordinarily approximately 10.sup.2 and is at most approximately 10.sup.4. Therefore, an ordinary photoelectric read-out means cannot accurately detect such a wide range of the amount of light emitted by the stimulable phosphor sheet. Even when a photoelectric read-out means is used which can detect such a wide range of the amount of light emitted by the stimulable phosphor sheet, because the wide range of the amount of emitted light must be detected, the change in the amount of emitted light per bit becomes large during A/D conversion of an image signal. Therefore, a small change in contrast cannot be detected easily. In general, the depth of a flaw occurring in a steep pipe, or the like, is very small, and therefore the difference between the flawed part and the surrounding parts in the radiation absorptivity is very small. In such cases, if a read-out image signal is obtained with a low contrast resolution, a difference between the flawed part and the surrounding parts in image density cannot be found easily in a visible radiation image reproduced from the read-out image signal.
The problems described above can be eliminated by compressing the range of the amount of light emitted by the stimulable phosphor sheet. For this purpose, a specific stimulable phosphor sheet may be used which is disclosed in, for example, Japanese Unexamined Patent Publication No. 63(1988)-214700, and whose sensitivity to radiation varies locally in accordance with the radiation absorptivity characteristics of an object. However, such a specific stimulable phosphor sheet is applicable to only a specific object and is difficult to manufacture. Therefore, the disclosed technique is not suitable for practical use.